This invention relates to variable capacity wobble plate compressors mainly adapted for use in air conditioning systems for automotive vehicles, and more particularly to an improved wobble plate compressor wherein the pressure in the crankcase is controllable for varying the displacement or capacity of the compressor.
A variable capacity wobble plate compressor in general is adapted to vary its displacement or capacity through a change in the angularity of the wobble plate. It is known e.g. from U.S. Pat. No. 3,861,829 to vary the refrigerant pressure in the crankcase for changing the angularity or angle of inclination of the wobble plate relative to the drive shaft. According to this patented invention, the compressor comprises a fluidtight housing, a drive shaft rotatably disposed in the housing, a cylinder block disposed in the housing and formed therein with a plurality of cylinders circumferentially arranged around the drive shaft and extending substantially parallel to the axis of the drive shaft, pistons received in the cylinders for reciprocating motions therein, and a wobble plate supported at its central portion by a trunnion pin extending perpendicularly to the drive shaft and axially movable therealong and also supported at its peripheral edge by a pivot rotatable about the drive shaft in unison therewith. The wobble plate is adapted to be pivotally displaced in unison with axial movement of the trunnion pin along the drive shaft to have its angularity varied relative to the drive shaft. As the wobble plate rotates in a position inclined relative to the drive shaft, the pistons are reciprocatingly moved in their respective cylinders for pumping actions. In this compressor, the resultant reaction force exerted by all the pistons on their compression strokes acts upon the wobble plate at a point inside a half portion of the circumference described by the axes of the cylinders, which is located at an opposite side of the drive shaft to the pivot point about which the wobble plate pivots, so that the wobble plate is permanently acted upon by the above resultant reaction force to become inclined relative to the drive shaft about the pivot point during the pumping actions of the pistons. The resultant reaction force of the pistons counteracts the pressure in the crankcase which acts upon the pistons as back pressure. Therefore, when there occurs a drop in the pressure in the crankcase, the wobble plate is displaced to increase the capacity in the angularity-increasing direction, whilst when there occurs an increase in the crankcase pressure, the wobble plate is displaced in the angularity-decreasing direction to decrease the capacity.
In the conventional wobble plate compressor described above, a diaphram valve is arranged across a line connecting the crankcase with a zone under lower pressure in the refrigerating circuit of an air conditioner, and operable in response to the refrigerant pressure in the line so that when a drop occurs in the line pressure due to a decrease in the thermal load on the refrigerating circuit, the valve is displaced to restrict the communication between the crankcase and the lower pressure zone in the refrigerating circuit to reduce the flow rate of blow-by gases which are leaked into the crankcase through gaps between the cylinders and the pistons, to the line, resulting in an increase in the crackcase pressure. Consequently, the angularity of the wobble plate decreases to reduce the displacement of the compressor. On the contrary, when the crankcase pressure rises due to an increase in the thermal load on the refrigerating circuit, the crankcase pressure decreases to cause an increase in the angularity of the wobble plate to obtain an increased displacement of the compressor.
In this arrangement, if the crankcase pressure is variable within a range from 5 to 10 percent of the difference between the suction pressure and discharge pressure of the compressor and the former is 30 psig, and the latter 200 psig, for instance, the crankcase pressure is limited within a range from 38.7 to 47 psig. Therefore, the conventional wobble plate control suffers from low responsiveness. This is particularly disadvantageous in the event that it is desired to suddenly reduce the displacement of the compressor, for instance when it is desired to temporarily relieve an automotive engine associated with the compressor of the compressor load to use the total engine output for driving the automotive vehicle on which the engine is installed, in accelerating the vehicle or when the vehicle is running on an ascending slope.
Furthermore, according to the conventional arrangement, the wobble plate angularity control also has low responsiveness to a change in the engine speed. That is, when a sudden change takes place in the engine speed, there occurs a change in the crankcase pressure due to a corresponding change in the amount of blow-by gases produced, followed by a change in the angularity of the wobble plate and also changes in the suction pressure and discharge pressure to vary the opening of the diaphragm valve, which determines the final crankcase pressure to set the wobble plate to a corresponding angular position.